Mirroring in Object Mode, taking rotation into account
If you are doing this so that they could be animated I would add a Copy Rotation Constraint and then invert the Y axis.

Or if you are modelling, Create a new collection with the left side (M) and then place a Collection Instance (Shift-A) of that and mirror the Instance. The collection instance has it's origin in the centre but it can be moved in the Context menu for the object -> Collections.

If you later on want to commit this to be real object you, just Make Instances Real (search for it with F3). They will still be linked copies.